Contiguous slotted and unslotted waveguide portions having substantially the same characteristic impedance



Aprll 21, 1970 3,508,175 CONTIGUOUS SLOTTED AND EGUIDE PORTIONS HAVINGSUBSTANTIA HE SAME CHARACT c IMPEDANCE Filed May 5. 1967 A. ALFORDUNSLOTTED WAV INVENTOR ANDREW ALFORD l5 Y' ATTZRNEYS United StatesPatent O M 3,508,175 CONTIGUOUS SLOTTED AND UNSLOTTED WAVE- GUIDEPORTIONS HAVING SUBSTANTIALLY THE SAME CHARA'CTERISTIC IMPEDANCE AndrewAlford, 120 Cross St., Winchester, Mass. 01890 Filed May 5, 1967, Ser.No. 636,478 Int. Cl. H03h 7/38; G01r 27/06 US. Cl. 333-33 7 ClaimsABSTRACT OF THE DISCLOSURE A slotted waveguide portion contiguous withan unslotted waveguide portion are arranged so that both havesubstantially the same characteristic wave impedance and propagationcharacteristics by making the slotted portion a little bit wider and/ora little bit shorter than the unslotted portion.

BACKGROUND OF THE INVENTION The present invention relates in general tocontiguous slotted and unslotted portions having substantially the samecharacteristic impedance and wave propagation characteristics especiallyuseful for making slotted waveguide measurements.

Slotted waveguides are widely used for making measurements at microwaveand higher frequencies. A probe penetrating through the slot normallysenses an indication of the electric field strength at points along thelength of the slotted waveguide to provide indications of the VSWR inthe slotted waveguide as a result of devices connected to the ends ofthe slotted waveguide. Although such slotted waveguides are useful formany applications, prior art devices themselves introduce enough ofamismatch to seriously limit the accuracy of the measurements being made.These problems are especially serious at the extremely short wavelengthswhen the width of the slot becomes comparable to a dimension of thewaveguide.

It is an important object of this invention to provide methods and meansfor enhancing the accuracy of slotted waveguide measurements.

It is a further object of the invention to achieve the preceding objectwith a slotted waveguide having a{ slotted portion that has its waveimpedance and propagation characteristics matched to that of theunslotted portion.

It is still a further object of the invention to achieve the precedingobjects over a relatively wide frequency range for the TE mode inrectangular waveguide.

It is still another object of the invention to achieve the precedingobjects with relatively little additional physical apparatus capable ofbeing reproducible when making slotted waveguides in productionquantity.

It is still a further object of the invention to achieve the precedingobjects with fixed structure that does not require adjustment fromslotted waveguide to slotted waveguide.

SUMMARY OF THE INVENTION According to the invention, there is awaveguide having a slotted portion with a longitudinal slot formed inthe waveguide wall intercoupled with an unslotted portion with the crosssectional area of the slotted portion slightly changed from that of theunslotted portion so that the impedance and propagation characteristicsof both the slotted and unslotted portions are substantially the same.

Numerous other features, objects and advantages of the invention willbecome apparent from the following specification when read in connectionwith the accompanying drawing in which:

3,508,175 Patented Apr. 21, 1970 FIG. 1 is a perspective view of oneembodiment of the invention in which a slotted portion of the waveguideis formed with its width slightly greater than that of the unslottedportion to effect the desired wide band impedance and propagationcharacteristics match;

FIG. 2 is a perspective view of another embodiment of the invention inwhich the slotted portion of the waveguide is slightly shorter than theunslotted portion to effect the desired wideband impedance andpropagation characteristics match; and

FIG. 3 is a perspective view of still another embodiment of theinvention embodying a stepped transition portion to elfect the desiredwide band impedance and propagation characteristics match.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference now tothe drawing and more particularly FIG. 1 thereof, there is shown aperspective view of an embodiment of the invention in which the slottedportion of the waveguide 11 is slightly greater than the unslottedportion 12. The width of the unslotted portion is a and that of theslotted portion a plus Aa while the height of both waveguides is b. Theslot width is d. The increase in width Aa is selected to be just enoughso that the impedance and propagation characteristics in both slottedportion 11 and unslotted portion 12 are substantially the same. Themanner of selecting this difference will now be described.

A waveguide slotted line, if made of the same size as the unslottedwaveguide in which measurements are to be made, has a highercharacteristic impedance than that of the unslotted waveguide. Let theunslotted waveguide impedance be Z and let the waveguide impedance ofthe slotted waveguide section (provided with a longitudinal central slotthrough one of the wider walls) be Z -j-AZ In general, thecharacteristic impedance of a waveguide is given by Equation 1.

where:

a=the internal width of the waveguide,

b=the internal height of the waveguide,

K=a constant, 7

f =the cutoff frequency of the TE mode, and f=operating frequency.

Since Aj=c where:

a=the velocity of light.

We may substitute the value of A in terms of 1 into Equation 2 and thusobtain the relationship between L, and a in which a=c/2j (4a) From theabove it follows that we can make up for the increase in f by M byincreasing dimension a by no as given by Equation 5 Aa/a=]cAf /2fDividing Equation 5 by Equation 4a we obtain Aa/a A When we wish todecrease Z by AZ, We may do this by decreasing the waveguide height b.The relationship be tween AZ, and Ab can be obtained from Equation 1 asReferring to FIG. 2, there is shown an alternate embodiment of theinvention in which the height of the slotted portion is reduced by Ab asexplained above to establish the characteristic impedance andpropagation characteristics of the slotted portion 13 and unslottedportion 14 substantially the same.

The dimensions Aa and Ab can be computed with the guidance of theequations set forth above and the dimensions trimmed experimentally tooptimize the matching of impedance and propagation characteristics.Alternatively, there exists an essentially experimental way fordetermining the parameters Aa and Ab. First, measure the cutofffrequency and impedance in an unslotted waveguide. Then cut a slot inthis waveguide of the desired width d and again measure the cutofffrequency and impedance. The first cutoff frequency and impedancemeasurements are the parameters f and Z respectively, and thedifferences between each first measurement and the corresponding secondmeasurement are AL, and A2 respectively, Aa and Ab may then bedetermined from Equation a and Equation 6. It may be desirable to effectpart of the compensation by changing the height and part of thecompensation by changing the width. This would involve an alteration ofthe cross sectional area of the slotted portion with respect to theunslotted portion so that the impedance and propagaion characteristicsof the two contiguous portions remain substantially the same.

Referring to FIG. 3 there is shown still another embodiment of theinvention in which the slotted portion 15 is coupled to the unslottedportion 16 by means including a transition slotted portion 17. Thespecific embodiment illustrated in FIG. 3 includes both height, widthand slot width compensation. Thus, the heights of unslotted portion 16,transition portion 17 and slotted portion 15 are respectively b, bAb andbAb The width of unslotted portion 16, transistion portion 17 andslotted portion 15 are respectively a, a+Aa and a+Aa The width of theslot in transition portion 17 is b and that of slotted portion 15 is dand d being greater than al Preferably, the electrical length oftransition portion 17 is an odd multiple of quarter guide wavelengths atthe center of the frequency at the desired operating band. Theprinciples set forth above are applicable to choosing the differentdimensions.

It is evident that those skilled in the art may now make numerous usesand modifications of and departures from the specific embodimentsdescribed herein without departing from the inventive concepts.Consequently, the invention is to be construed as embracing each andevery novel feature and novel combination of features present in orpossessed by the apparatus and techniques herein disclosed and limitedsolely by the spirit and scope of the appended claims.

4 What is claimed is: 1. High frequency apparatus comprising, meansdefining a slotted waveguide portion formed with a slot of predetermineddimensions along the direction of energy propagation, means defining anunslotted waveguide portion contiguous with said slotted waveguideportion, said slotted and unslotted waveguide portions beingcharacterized by finite cutoff frequencies and having different crosssectional areas with means for establishing the relationship betweensaid different cross sectional areas so that the impedancecharacteristics and cutoff frequencies of said slotted and unslottedportions are substantially the same, whereby said waveguide portions ofdifferent cross sectional areas coact with said slot of predetermineddimensions to form a waveguide having substantially the same impedanceand finite cutoff frequency along its length. 2. High frequencyapparatus in accordance with claim 1 wherein said slotted and unslottedwaveguide portions are dimensioned to propagate the TE mode.

3. High frequency apparatus in accordance with claim 2 wherein saidcross sectional areas are rectangular.

4. High frequency apparatus in accordance with claim 3 wherein the widthof said slotted portion is slightly greater than that of said unslottedportion.

5. High frequency apparatus in accordance with claim 3 wherein theheight of said slotted portion is slightly less than that of saidunslotted portion.

6. High frequency apparatus in accordance with claim 4 wherein theheight b of said slotted portion is slightly less than that of saidunslotted portions by a difference Ab to establish substantially thesame empedance Z in said slotted and unslotted portions and the width aof said slotted portion is Aa greater than that of said unslottedportion to establish substantially the same cutoff frequency i in saidslotted and unslotted portions.

7. High frequency apparatus in accordance with claim 1 wherein saidslotted and unslotted waveguide portions have a common axis and saidslot is along the length and generally parallel to said axis.

References Cited UNITED STATES PATENTS 2,433,368 12/1947 Johnson.

2,454,530 11/1948 Tiley 3333l 3,005,168 10/1961 Fye 333-31 3,050,6898/1962 DeLoach 33333 HERMAN K. SAALBACH, Primary Examiner C. BARAFF,Assistant Examiner US. 01. X.R.

